Located at the USDA-ARS J. Phil Campbell, Sr., Natural Resource Conservation Center in the Southern Piedmont of Northeast Georgia, the study site is a small, ∼100 ha, watershed that consists of grazed pastures, a cropped (2·7 ha) catchment under conservation tillage that is amended annually with poultry litter, and a wooded riparian zone from which agricultural animals are excluded (Fig. 1). With the exception of the cropped catchment, rotational grazing of a purebred herd of Angus cattle between 500 and 700 head occurred on the paddocks on both sides of the stream flowing into the pond (Fig. 1). These paddocks comprise 56 ha upstream from the pond. No more than 200 head of cattle (cows plus calves) were grazed on a paddock at one time. The average total cattle days (number of cattle × number of days on pasture) for both years was 2398. Cattle were moved between paddocks every 6 weeks depending on the availability of forage. An unknown number of deer, rodents and other forms of wildlife, a diverse avian community, and domestic animals from the neighbouring subdivision were potential sources of faecal bacteria. Topographic slopes range from 2% to 10% in the pastures and cropped field. In the riparian zone, stream banks can be as steep as 30%. Bedrock begins 3–30 m below ground surface. Soils belong to the Cecil and Pacolet series (fine, kaolinitic, thermic, Typic Kanhapludult). A first-order stream fed by a series of springs flows into and out of a pond that captures base and storm flow from approximately 60% of the 100-ha watershed. The remaining 40% of the watershed that the pond does not serve feeds into the stream below the pond. The bathymetry of the pond was established through GPS/GIS survey, and is approximately 225 m long, and has an average width of 70 m. The deepest part is 4 m from permanent pool level and occupies a 35- by 80-m2 area close to the outlet. The bed level gradually rises towards the edges where it is 0·4 m from permanent pool level. The pond holds about 24 000 m3 of water at permanent pool level. Outflow is through a riser and horizontal conduit pipes. A 120° V-notch weir and a 0·46-m H-flume of USDA specification (Brakensiek et al. 1979) are used to measure inflow 50 m upstream of the upper end of the pond and at outflow, respectively (Fig. 1). Pond inflow also occurs from a small spring one-third of the way from the upper end of the pond and passes through another small pond before entering the pond, but the small pond is not instrumented for measuring flow (Fig. 1). Differences in measured pond inflow and outflow rates partly indicated the contribution of the small pond. Appropriate calibration curves were used to convert flow head at each measuring device into flow rate. The V-notch weir head was measured with a Campbell Scientific Inc. (Logan, UT, USA) shaft encoder with the float mechanism installed in a stilling well. For the flume, a 17·24-kPa depth sensing transducer (Druck Incorporated, New Fairfield, CT, USA) located in a stilling well of the flume was used. Each sensing device was connected to a Campbell Scientific CR10X data logger programmed to record average flow depth every 5 min. These 5-min flow data were then processed with a computer to produce flows at appropriate time intervals needed for the research. Pond depth was similarly measured with a shaft encoder. Throughout each year, baseflow samples were collected every second Tuesday of each month. Because of logistical considerations, E. coli O157:H7 was not enumerated every month. Pond inflow and outflow samples were collected at the lip of the V-notch weir and H-flume, respectively. Pond water samples were taken from the surface at a designated site within the pond. Because of the low concentrations of E. coli O157:H7 that Loge et al. (2002) reported, 20-l samples were taken at the inflow and outflow sites and within the pond (Fig. 1). Three pairs of 10-l Nalgene containers, sterilized with 5% commercial hypochlorite and rinsed with ultrapure water, were used for collecting baseflow samples for E. coli O157:H7. Sterile 1·5-l Nalgene bottles were used to collect samples for total suspended solids (TSS), faecal indicator bacteria and total direct microbial counts. Subsamples of 100 ml were taken for TSS determination. Because of the previously observed log-normal distribution (Fisher et al. 2000), single 0·2- to 0·4-l subsamples were taken for determination of faecal indicator bacteria. Subsamples as 9-ml undiluted aliquots were taken for total direct microbial counts. Flux rates of microbes were determined by the product of microbial concentration (MPN per litre of water) and the rate of the pond inflow and outflow (l h−1) at the time of sampling.
Figure 1. Map of watershed on USDA-ARS J. Phil Campbell, Sr, Natural Resource Conservation Center (, Road; , stream; , surface water; , forested and / or Riperian; , Grazingland; , Hayed; , Grass (not Grazed); , Research watersheds).
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